It will depend on how Oda handles Sommer's steel heart.
Sommer's heart seems to be linked to is immortality. When Zoro talked about destroying it he freaked out. Yet, it seem to be incredibly durable and hard to damage. Sommer took a hit for one of Luffy strongest attacks that destroyed his body but not his heart. That's would only be possible if his heart was made from Imu's omen black flames that can turn into steel. Imu clas6h against the magical hammer squirrel with that power and and the hammer couldn't even chip Imu's steel.
Of course, the question still remains all HK have steel hearts or is that something unique to Sommer. We know HK can be killed. Loki killed Harald.
If the steel hard is unique to Summer and it's made from Imu's omen black flames then Zoro will need to master ACoC to destroy the heart and kill Sommer. If all HK have steel hearts and Luffy failed to destroy it for plot I can see Zoro only needing to gain control but not fully master ACoC to beat Sommer.
It could be sommers way of protecting himself as well.
Paramacia type can change their body property like Doflmaingo stiching up his own organ after being damaged.
So that heart could be thorne related, or some property devil fruit to save Sommers at last minute.
HK harald was killed with Lightening + AdCoC.
Sommers got hit by Luffy AdCoC + Lightening + Gear 5.
Realistically he shouldn't be alive.
As far as his heart is intact, he would always regenerate. That's the kind of cope oda is pulling with sommers most likely.
All the zoro opponent have some Weird hax, King had Lunarian ability, Pica had Stone ability undefeated if he is touching the ground.